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I have a Perfect Fit air filter that was put in when I replaced my AC with a Trane Heat Pump last year. It defintely works better than the previous one, but the replacement filters are hard to find, relatively expensive, and are needing to be replaced quite often. They are 5 inches thick.

Is 5 inches thick better than the standard 1 inch thick filters? If so, can you explain?

If not, do you think I could just put a frame inside that would close down the opening to be 1 inch thick to use more standard filters and not lose in performance or worry about problems with it.

5" filters definately better than 1". average house we have them in filter life is 1 year. how often are yours plugging up? might have duct problem pulling in air from dirty area. what is exact filter size,may be other companies that make filter to fit your housing that would be easier to find.

Thanks for your replyl,
Some more information...
I had my ducts cleaned about the same time I had the new filter put in so my ducts are pretty good. The filters need to be replaced every 2 months and are costing about $50 a piece. I don't mind changing a $10-$20 filter every 2-3 months, but $50 every other month is getting expensive. Any suggestions are appreciated. The size is 23.5 x 21 x 5.

that is an oddball filter size probably why so pricy. still believe problem may be infiltration of return duct unless house is full of pets. might want to get looked at to convert to a more standard size filter.

I think a 5" filter will last at least 6 months and should last a year unless you do a lot of sanding in your house. $50 seems about right from the places I checked on-line, definitely a little pricey.

I have a Carrier 5"x20x25, about $32. I take it out at 6 months, vacuum it, and replace at 12 months.

Did you look inside your ducts after they were cleaned?
I have seen duct cleaners just disturbe the dust not remove it, then ending up in your filter. Best thing to do is borrow a camera and see for your self, and if needed the guys who took your $$$.

Filters pulugging up quickly is not caused by dirty ducts. Leaking ducts will cause filters to get dirty faster, as will a number of other things.

If your Perfect Fit filters are getting dirty that fast, you have a problem. Could be return leaks pulling in air from dirty places, could be supply leaks causing lots of infiltration in the house, old carpet, pets, dust being swept into the return during house cleaning, vaccume cleaner blowing dust in the air, etc.

There is also a very good chance that the filter system isn't properly sized for the CFM of the airflow system. The more airflow being moved through the system, the larger the filter system must be.

The frequency at which a filter loads up with dirt is typically determined by:

1. The dirt holding capacity of the filter.
2. The cubic feet per minute of air moving through the system.
3. The amount of dirt in each cubic foot of air.

#1 is determined by the cubic inches of surface area of the filter media. The larger the filter frame size, the more area there is to provide additional media surface area within any filter of a given thickness. In pleated filters the extra thickness of the filter allows them the room to provide more pleats of filter media, thus giving them more surface area. This doesn't mean that all 5" thick, 20 x 25 filters have the same amount of filter media surface area. Some of them cheap out and put in less pleats, which results in the media becoming saturated with dirt faster. The Aprilaire Spaceguard has 76 cubic *feet* of filter media contained in a 20 x 25 filter.

If you take x amount of dirt and distribute it over a much larger area, it will take much longer to block the airflow.

#2 the cubic feet per minute of air moving through the system is determined by the BTU output of the heating and cooling system. The higher the BTU of the system is, the more CFM the blower system is sized to deliver. The filter system should be sized for the CFM output of that blower system. The more CFM there is, the larger the filter must be. Even the Spaceguard 20 x 25 filter will load up much faster if it's put in a 5 ton system with 2,000 CFM of air because you're pulling more cubic feet of air through it, and *each cubic foot contains x amount of dirt*.

If you have the fan switch on the thermostat in the "on" position, it runs the blower continuously, so more cubic feet of air per day are being pulled through the filter.

OR

If you turn the thermostat to a very low temperature you're increasing the cubic feet of air per day going through the filter.

#3 The amount of dirt contained in each cubic foot of air varies depending on how much dust is infiltrating the house and how many particles are being released by things in the house. Even humans and pets give off particles. Animals that shed are a big problem too.